b. in South Shields, 13 September 1883, the third son and fifth child of Christmas and Cecilia (née Thornton) Thomas. His father was a farmer's son from the Narberth district of Pembrokeshire who went to sea, and by the time Percy Edward was born he was captain of a sailing vessel. His mother came from Wedmore, Somerset. When the son was ten years old the family moved to Cardiff, attracted, like other similar families, by the flourishing coal trade. He was educated privately at Hasland House until his father died in 1897, when he was moved to Howard Gardens high school; but he had seen the world from a tender age with his father every summer and visited cities such as St. Petersburgh, Odessa, Istanbul, Genoa, Fiume and other ports. His early experiences must have coloured his later career. His father died at sea and was buried at Leghorn. His mother found the boy a place in a ship's office in Cardiff, but the work did not appeal to him. The vicar of Llandough took him to a phrenologist who concluded that he was suited to be an architect. He took articles in the office of E.H. Burton F.R.I.B.A. for five years, but by the fifth year he felt he should be paid, and accepted five shillings a week rather than be released from his articles. In a competition to design a school in Cardiff his entry was deemed better than that of his master, and he also won the architecture competition in the national eisteddfod at Llanelli in 1903. In January 1904 he was given a post in Leigh, Lancashire, by J.C. Prestwich, but moved after a year to Bath as an assistant to R.A. Brinksworth. After two years he was searching for a more responsible post and answered an advertisement which had a box number only. To his surprise he found himself back with J.C. Prestwich, but as a chief assistant. He moved again in 1906 to Henthorne Stott in Manchester. He collaborated with Ivor Jones of Cardiff in open competitions, and in 1911 they won the prize for designing a technical college in Cardiff. This gave him the opportunity to return to Cardiff in 1913 in partnership with Ivor Jones. His career was interrupted by World War I, when he joined the Artists’ Rifles at the end of 1915. He was commissioned in the 210 Field Co., R.E. and found himself at the Somme, not in Egypt as he had hoped. He was promoted Staff Officer R.E. XIII Army Corps, was twice mentioned in despatches and won the military O.B.E.

Released from the army in February 1919, he returned to his work in Cardiff. In the period between the wars he won many competitions for important commissions, e.g. police headquarters and fire station, Bristol, 1924, Newcastle, 1925 and Accrington, 1930; civic centres in Swansea, 1930, Tunbridge Wells and the town hall at Swinton and Pendlebury, 1934, the last two in collaboration with Ernest Prestwich of Leigh. In addition to these successes he had important commissions which included county offices for Glamorgan, the Temple of Peace — directly commissioned by Lord Davies — both in Cathays Park, Cardiff; county offices for Carmarthenshire and police headquarters and fire station for Worcester. He was now a recognised authority on planning and designing public buildings, and he was appointed planning consultant to Cardiff, Aberdeen, High Wycombe, Blackpool, and the Royal Borough of Kensington, as well as to the county councils of Flintshire and Shropshire. In 1935 he was invited to design the new campus of University College Aberystwyth and he was the architect of many of its buildings: the agricultural research laboratory, the swimming pool (where Forest of Dean stone was used against his advice) and the dairy science building. In 1935 the L.M.S. railway company asked him to redesign Euston station, but the outbreak of war put paid to these plans.

He was elected President of the Royal Institute of British Architects in 1935, and in 1939 was awarded their royal gold medal, one of the few architects to receive both honours. They awarded him the R.I.B.A. bronze medal in 1930, for James Howell's new shop in Cardiff, in 1935 for the Swansea Civic Centre, in 1938 for Swinton and Pendlebury Town Hall and in 1939 for the Temple of Peace. He was elected an honorary corresponding member of the American Architectural foundation in 1936, and was also President of the French and British Architectural Union. The partnership with Ivor Jones was dissolved in 1937 by mutual consent and he worked on his own until 1946 when he took his son Norman into partnership, and in 1952 enlisted William Marsden and Wallace Sweet.

World War II halted all public works and during this period Percy Thomas was immersed in government service in Wales, and continued to be so until the end of the 1950s. In 1940 the Lord Lieutenant of Glamorgan invited him to be regional officer in the Ministry of Supply, and when this ministry was taken over by Lord Beaverbrook he was made director. When the Ministry of Production was set up in 1942 he became regional director and chairman of the Welsh region, a post he held while the ministry existed. Sir Stafford Cripps invited him to continue as an independent chairman of the Welsh board and as a member of the National Productivity Advisory Council. He became one of the best-known figures on the Welsh industrial scene and was in great demand as chairman of meetings and committees on benefits and the problems of modern industry. Immediately after Anthony Eden's appeal for volunteers he joined the Local Defence Volunteers and was assistant to Col. Otto Jones in recruiting in Glamorgan. Later he was appointed a Lt. Colonel and Commander of the 22nd Battalion of the Glamorgan Home Guard. In 1943 he was elected president of the R.I.B.A. for the second time, the only precedent being Sir William Tite in 1867. He served the council for two years and was elected for a further year in 1945, thus holding the presidency for five years. He was mainly responsible for reconciling the interests of private practitioners and the official body of architects, who were by now in the majority, and in this way, incidentally, transformed the establishment from being London -based to being truly national.

He was responsible for much industrial design in post-war Wales, e.g. British Nylon Spinners great factory at Pontypool, new power stations at Aberthaw. As consultant architect to the Steel Company of Wales he was responsible, with W.A. Atkins, for the gigantic strip mills in Aberafan and Llanelli. He was consultant architect to the Ministry of Transport on numerous projects in Wales after the war, e.g. the Conwy by-pass bridge, the Severn bridge, and by-passes at Newport and Neath. He was also consultant architect to the Cardiff hospitals, the universities of Nottingham, Bristol and Wales, all British Electricity Authority's activities in Wales, and the Federation of British Industry's new headquarters in London.

He was an assessor on many major architectural competitions during this time, the most important being the T.U.C. headquarters in Great Russell Street and Coventry Cathedral. As consultant to the hospitals he was an assessor for the medical centre at the Heath in Cardiff. In 1952 he was appointed honorary colonel in the 109 Army Engineer Regiment (Glam.) R.E. (TA), a regiment in which his son was a senior officer.

He married Margaret Ethel, daughter of Henry Turner of Penarth, in 1906, and they had one son and three daughters. She died in 1953. In 1961 he was seriously ill, and after a brief period as consultant to the practice, he retired in 1963. His health was fragile for the rest of his life, and he died 19 August 1969.

Besides his own specialised field, society benefited greatly from his brilliant services, and he received many honours, including a LL.D. from the University of Wales in 1937, and he was awarded a knighthood in 1946. He was a J.P. in Cardiff from 1946, Deputy Lieutenant for Glamorgan from 1945 and High Sheriff 1949-50. He edited a four volume work, Modern Building Practice, in 1936-7, and published an interesting autobiography, Pupil to President, privately in Leigh-on-Sea in 1963. It is said that his architectural style had three special qualities: symmetry, simplicity and sincerity.